This project is directed to an assessment of the energy sources (types and amounts of metabolites) used by the kidney in vivo to support its several functions. To have any true physiological relevance, the studies must (1) be carried out on the intact functioning kidney and (2) be carried out at normal endogenous blood concentrations of the several metabolites. These two criteria necessitate studies on the dog and the use of C14-labeled metabolites. In essence, a single C14-labeled metabolite is infused intravenously at a constant rate. A renal venous catheter and an arterial needle permit sampling of blood leaving and blood entering the kidney. The extraction of the C14-labeled isotopic and of normal cold metabolite is measured. The production of 14CO2 and of total cold CO2 by the kidney is measured simultaneously. These data when analyzed indicated the total amount of CO2 produced and the proportions of CO2 derived from the labeled metabolite. This method has been used in one paper published by the principal investigator and in one study in progress. It is feasible.